The material requirements for gas turbine engines are continually being increased. Components formed from powder metal gamma prime precipitation strengthened nickel-base superalloys can provide a good balance of creep, tensile and fatigue crack growth properties to meet these performance requirements. Typically, a powder metal component is produced by some form of consolidation, such as extrusion consolidation, then isothermally forged to the desired outline, and finally heat treated. These processing steps are designed to retain a very fine grain size within the material. In order to improve the fatigue crack growth resistance and mechanical properties of these materials at elevated temperatures, these alloys are then heat treated above the gamma prime solvus temperature (generally referred to as supersolvus heat treatment), to cause significant, uniform coarsening of the grains.
However, if conventional manufacturing procedures involving hot forging operations are used to form relatively small components such as high pressure compressor blades and vanes, and fasteners, then a wide range of local strains and strain rates are introduced into the material. This results in non-uniform critical grain growth during post forging supersolvus heat treatment. Critical grain growth is defined as localized abnormal excessive grain growth to grain diameters exceeding the desired range, which is generally between about ASTM 2 and ASTM 9. (Reference throughout to ASTM grain sizes is in accordance with the standard scale established by the American Society for Testing and Materials.) More specifically, for powder metal alloys, the desired range is about 0.0006 inch (ASTM 9) to about 0.007 inch (ASTM 2); for cast and wrought alloys it is about 0.002 inch (ASTM 6) to about 0.020 inch (ASTM 00). This non-uniform critical grain growth may detrimentally affect mechanical properties such as tensile and fatigue. Therefore, large grains of this size are to be avoided, particularly within relatively small components such as high pressure compressor blades and vanes, and fasteners.
U.S. Pat. No. 4,957,567 to Krueger et al., assigned to the same assignee of the present patent application, eliminates critical grain growth in fine grain nickel-base superalloy components by controlling the localized strain rates experienced during the hot forging operations. Krueger et al. teach that, generally, local strain rates must remain below a critical value, .epsilon..sub.c, in order to avoid detrimental critical grain growth during subsequent supersolvus heat treatment. Strain rate is defined as the instantaneous rate of change of strain with time.
Accordingly, based on the teachings of Krueger et al., it was believed that in order to produce a uniform grain size after post-forging supersolvus heat treatment, the strain rate experienced during hot working must never exceed the critical value, .epsilon..sub.c. However, this is not always a practical alternative.